1. Field of the Invention
This invention relates to a semiconductor laser apparatus which attains laser oscillation with a stabilized oscillation wavelength.
2. Description of the Prior Art
Semiconductor laser devices which are mass produced can attain laser oscillation at a low threshold current and obtain considerably satisfactory results in characteristics such as the fundamental transverse mode, the single longitudinal mode, long life, etc., but they have problems with regard to a stabilized oscillation wavelength (i.e., the stabilized longitudinal mode) in that the oscillation wavelength varies continuously or discontinuously depending upon a variation in temperature and/or current, resulting in optical output noise which is noticeable when the laser devices are exposed to light and/or a reflected laser light from the laser devices.
In order to eliminate the above-mentioned problems and to stabilize the longitudinal oscillation mode, the suppression of mode hopping (i.e., changes in the longitudinal oscillation mode) has been tried over a wide range of temperatures by the following approaches:
The first approach for the suppression of mode hopping is the use of diffraction grating type lasers such as distributed feedback (DFB) lasers, distributed Bragg reflector (DBR) lasers, etc., which have diffraction gratings in the waveguide. These laser devices are excellent in wavelength selectivity due to the periodicity of the diffraction gratings, so that they can attain the stabilization of the longitudinal mode over a wide range of temperatures. However, their production process is complicated and it is difficult to use some semiconductor laser materials (e.g., materials which are readily oxidized).
Second, compound resonator type lasers including cleaved coupled cavity lasers (in which two semiconductor lasers are coupled by their facets and/or which are separated into two laser-operating areas by an etching technique) are used. The two semiconductor lasers operate independently, resulting in the synchronization of their wavelengths, making possible the stabilization of the longitudinal mode. However, their operation relies upon the skill of skilled workers, and moreover small changes in the spacing between the two laser devices cause changes in the longitudinal mode, resulting in optical output noise.
Third, the reflective index of each of both facets of a semiconductor laser device is made high to increase the internal optical density of the device, resulting in the suppression of the non-oscillation mode, thereby attaining stable oscillation in a longitudinal mode notwithstanding changes in temperatures.
Fourth, the concentration of Te, etc., to be doped into the n-cladding layer of a semiconductor laser device is set at a high level resulting in a supersaturated absorber by which loss-gratings are formed, whereby the suppression of the non-oscillation mode is attained and stable laser oscillation is attained in a longitudinal mode notwithstanding changes in temperatures.
Although the third and fourth approaches provide stable oscillation in a longitudinal mode regardless of changes in temperatures to a certain extent, mode hopping occurs not only when temperatures rise beyond a certain value, but also when temperatures fall beyond a certain value. That is, mode-hopping hysteresis occurs, causing a limitation in the range of temperatures and/or currents used.
Fifth, internal reflector interferometric lasers having a reflecting area inside thereof are used. These laser devices have a waveguide which provides one or more regions having different effective refractive index in which reflection occurs resulting in an interference effect. Such an interference effect results in the stabilization of a longitudinal mode. In order to create a difference in the refractive index, the thickness of the active layer must be made uneven and/or semiconductor crystal materials having a different composition ratio must be used for the formation of the laser devices.
The sixth approach is that two or more waveguides having different effective cavity lengths are optically coupled therebetween, resulting in an interference effect, which makes possible the stabilization of a longitudinal mode. Examples of these laser devices are OEMI lasers (I. H. A. Fattah et al., Appl. Phys. Lett., 41(2), 112, 1982) etc.
Although the fifth and sixth approaches allow the stabilization of a longitudinal mode at a temperature in the range of several degrees to several tens of degrees, it is impossible to stabilize the longitudinal mode with reproducibility over a wider range of temperatures. That is, although the suppression of a limited number (e.g., 4 or 5) of the longitudinal modes which are adjacent to the oscillation longitudinal mode is possible, the internal reflectivity of these laser devices are so insufficient that a greater number of adjacent modes cannot be suppressed, making it difficult to put them into practical use.
Seventh, external resonator type lasers are used, wherein laser light emitted from one of the facets of the laser device is reflected by an external reflector to return to the facet of the laser device, resulting in an interference effect between the external mode based on the distance from the facet of the laser device to the external reflector (i.e., the external cavity length) and the longitudinal mode based on the distance from one facet of the laser device to the other thereof (i.e., the internal cavity length of the laser device). The stabilization of a longitudinal mode can be attained by utilizing such an interference effect. The selectivity of the longitudinal mode in the seventh approach depends upon the external cavity length and the amount of reflected light. In order to increase the amount of reflected light, a semiconductor laser apparatus shown in FIG. 3 has been proposed, wherein a lens 2 is disposed between the laser device 1 and the external reflector 3 in such a manner that laser light emitted from the laser device is incident upon the external reflector 3 through the lens 2 and then reflected to return to the laser device 1 through the lens 2. The laser device 1 is fixed on a mounting base 4. Since the external cavity length (L) unavoidably becomes long due to the above-mentioned structure, the external mode interval .DELTA..lambda.e (=.lambda..sub.0.sup.2 /2 L, wherein .lambda..sub.0 is the central wavelength) becomes small, so that stable oscillation in a longitudinal mode cannot be attained. Although laser light emitted from the laser device can be returned to the laser device by a concave reflector, the production process of such a laser apparatus is complicated and it is difficult to position the facet of the laser device at the center of the concave reflector.
The inventors of this invention designed a semiconductor laser apparatus in which a plane reflector is positioned to face one facet of the laser device in a parallel manner at the distance of 100 .mu.m therebetween (i.e., with an external cavity length of 100 .mu.m), and stable laser oscillation was attained in a longitudinal mode over a range of about 10.degree. C. In order to further expand the temperature range, the inventors tried to shorten the external cavity length and measured changes in the oscillation wavelength when temperatures are changed at an optical output power of 3 mW, resulting in the characteristic curve (FIG. 4) showing the relationship between the temperatures and the oscillation wavelengths. FIG. 4 indicates that mode hopping successively occurs from one longitudinal mode to the adjacent longitudinal mode in each of the A and B zones of oscillation wavelengths, which makes the stabilization of laser oscillation in a longitudinal mode impossible. More particularly, FIG. 4 indicates that mode hopping successively occurs from the initial longitudinal mode to the adjacent longitudinal mode at the .circle.a points at which the temperature rises and the other mode hopping occurs to return to the initial longitudinal mode at the .circle.b points at which the temperature falls. That is, mode hopping hysteresis phenomenon arises with changes in temperature.
As mentioned above, semiconductor lasers which stably oscillate in a single longitudinal mode over a wide range of temperatures and which are readily produced have not yet been proposed at present.